1. Field of the Invention
The present invention relates to a method of preparing a high softening point pitch which can be used as a precursor when coating graphite for a negative electrode of a lithium ion battery and preparing a functional carbon material, such as spherical active carbon and isotropic pitch based carbon fiber, and the high softening point pitch prepared by the method.
More particularly, the present invention relates to a method of preparing a high softening point pitch having a high softening point as a polyene radical intermediate is formed and an alkylaromatic radical material is linearly linked to the polyene radical intermediate in a benzyl or methylene form using the polyene radical intermediate and polymerized, and the high softening point pitch prepared by the method.
2. Description of the Related Art
Carbon fibers used as fillers for polymer compositions (carbon fiber reinforced plastics; CFRP) are mainly classified into polyacrylonitrile (PAN) based carbon fibers and pitch based carbon fibers according to source materials. Among them, the pitch based carbon fibers are classified into mesophase pitch based carbon fibers and isotropic pitch based carbon fibers according to the types of pitches serving as precursors. The mesophase pitch based carbon fiber is prepared using a mesophase pitch having an optical property of anisotropy as the precursor, and the isotropic pitch based carbon fiber is prepared using an isotropic pitch having an optical property of isotropy as the precursor.
Regarding the physical property of the pitch based carbon fiber, the mesophase pitch based carbon fiber generally represents high strength, high elasticity, and low elongation, while the isotropic pitch based carbon fiber represents low strength, low elasticity, and high elongation.
Meanwhile, although the mesophase pitch based carbon fiber represents high strength and high elasticity, the mesophase pitch based carbon fiber has the elongation of less than 0.7% to represent compressive strength lower than that of the PAN based carbon fiber. Accordingly, the mesophase pitch based carbon fiber has a limitation in the application to high elasticity and high thermal conductivity materials.
In contrast, although the isotropic pitch based carbon fiber represents high elongation and low price, the isotropic pitch based carbon fiber does not represent high strength of 1 GPa or more. In order to use the carbon fiber for the CFRP having both of high strength and high elongation by improving low tensile strength of the isotropic pitch based carbon fiber, there have strongly been required a new design of a molecular structure of an isotropic pitch used as a precursor of the carbon fiber, a physical design of a pitch having a high molecular weight and an effective melting characteristic, and the studies on preparation processes.
The low tensile strength of the isotropic pitch based carbon fiber may result from low molecular orientation. Since a PAN precursor, which is the same optical isotropic material, is subject to an additional melt-stretching process after a wet spinning process to be stretched to 10 times or more to form the molecular orientation, a stretching-stabilizing process is required to perform carbon-fiberization while maintaining the molecular orientation of the stretched PAN fiber.
Since a mesophase pitch precursor obtains high molecular orientation by shear stress in spinning due to a high molecular stacking property, the mesophase pitch precursor may obtain the high molecular orientation without the stretching-stabilization process. However, the mesophase pitch precursor has low elongation because of forming a carbon fiber structure in which larger domains of the mesophase pitch are linearly linked to each other.
The isotropic pitch, which serves as a precursor of the isotropic pitch based carbon fiber, may not smoothly obtain molecular orientation in spinning due to isotropic molecular orientation, a low stacking property or a non-stacking property, and a small domain structure. In particular, since pitch fiber obtained by fiberizing the pitch has a significantly brittle characteristic due to a lower molecular weight of the pitch, the pitch fiber may not obtain the same molecular weight as that of the PAN precursor through a post-stretching process. Accordingly, the pitch fiber must be stretched to a target fiber (generally, less than 2 cm) in the melting state that short die swell is maintained in a melt-spinning process, in order to obtain the molecular weight. When the design, the preparation, and the preparation process of the precursor pitch allowing molecular orientation are developed through an innovative molecular structure design, the isotropic pitch based carbon fiber having higher molecular orientation may be prepared.
In addition, the pitch based carbon fiber is prepared in the following manner. A pitch serving as a precursor is melt-spun using melt-spinning machine in the temperature range of at least 50° C. higher than a softening point and fiberized. Thereafter, the fiberized pitch is oxidized in the temperature range of a glass transition temperature of the pitch to 320° C. at an oxidation atmosphere for a predetermined time to be subject to a curing process. Then, the result is processed in the temperature range of 800° C. to 3000° C. at an inert atmosphere or a vacuum atmosphere according to uses and preparation prices. Particularly, in the carbon fiber, since the prices of the PAN and the pitch serving as the precursors occupy 40% of the whole preparation costs, when a precursor having a high carbonization yield is prepared at a low price, the price of the carbon fiber may be effectively cut down as generally known in the art.
To prepare a high softening point isotropic pitch having a softening point of at least 200° C. as a source material of the isotropic pitch based carbon fiber, an alkylaromatic condensate, a condensed aromatic compound, which is a residue of a coal and petroleum based byproduct, serving as the mixture thereof, or a material formed by distilling the alkylaromatic condensate may be subject to heat treatment and distilled at an atmospheric condition, a pressurized condition, and a vacuum condition to condensation-polymerize low molecular weight components, and to remove volatile components (Korean Patent Publication No. 1993-0005526). In addition, a source material component may be condensation-polymerized using a catalyst, such as AlCl3 or HF/BF3, and changed to a polymer component (Japanese Patent Publication No. 1998-83814). In addition, source molecules may be linked to each other by halogenation dehydrohalogenation by adding a halogen group (Cl2 or Br2) for polymerization (Korean Patent Registration No. 10-0244912 and Korean Patent Registration No. 10-0305372).
However, it is difficult to prepare high softening point isotropic pitches in similar molecular structure having relatively similar physical properties and having a narrow molecular weight distribution from various source materials which are various molecular mixtures having various kinds of reactivity through the simple heat treatment among the above methods.
The pitch prepared using AlCl3 and HF/BF3 through cationic polymerization to make a relatively easily controllable reaction may have a higher softening point, a higher yield rate, and narrower molecular weight distribution when comparing a pitch prepared through simple heat treatment. However, according to the preparation method using AlCl3 as a catalyst, the removal of AlCl3, which is a solid component, may be difficult and require great costs. In addition, according to the preparation method using HF/BF3 as a catalyst, the catalyst need not be removed. However, since the catalyst has a significantly high corrosion property, great costs may be required for the process and the management of preparation facilities.
According to the method of linking source molecules to each other by halogenation dehydrohalogenation by adding the halogen group (Cl2 or Br2) for the polymerization, a high softening point pitch may be prepared at a higher yield rate under a lower temperature. However, when Cl2 is used, there are the high corrosiveness of Cl2 and the difficulty in handling of Cl2, a reaction tube may be blocked due to the production of solid organic chlorate resulting from the excessive chlorine reaction, and great process costs may be required to remove an HCl component which is generated in excessively large amount. When Br2 is used, problems may occur similarly to those of Cl2, and the price of Cl2 may increase the preparation cost of the high softening point pitch.
Recently, as researches and studies have been actively performed to use petroleum refining residues, which have an economical price, an excellent elastic modulus, and excellent thermal and electrical conductivity, as source materials of the isotropic pitch based carbon fiber, fluidized catalytic cracker (FCC), decant oil (DO), fluidized catalytic cracking decant oil (FCC-DO), and pyrolysis fuel oil (PFO) are used as the source materials, and dicumylperoxide (DCP), benzoyl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, and methyl ethyl ketone peroxide, which are organic polymer radical initiators, may be used as polymerization initiators to prepare pitch for carbon fiber using pyrolysis fuel oil (PFO) (as disclosed in Korean Unexamined Patent Publication No. 10-2015-0005382). However, radicals may be non-radicals or volatilized to be removed at the temperature of at least 250° C. that condensation polymerization is mainly made. Accordingly, even if a large number of initiators are used, the high degree of condensation may not be obtained.